Railway car brake systems generally include a pneumatic brake control valve responsive to relative changes in brake pipe pressure. When brake pipe pressure drops below a predetermined value, the brakes are applied. Alternatively, when the brake pipe pressure increases over the predetermined value, the brakes are released. Prior art brake control valves generally include a service portion and an emergency portion. Each section includes a separate piston responsive to brake pipe pressure on one side and a reference pressure on the other side.
Control valves are designed to be insensitive to pneumatic noise signals in the brake pipe that would cause undesired emergency braking and undesired brake release. Various mechanisms have been used, for example, chokes or restrictions, for placing the brake pipe and the reference chambers in fluid communication to compensate for the pneumatic noise. In one instance, a small stability choke provides fluid communication between the auxiliary reservoir and the brake pipe. Prior art stability chokes are bidirectional and thereby require additional structure to only allow airflow in one direction. The additional structure complicates the manufacture and operation of the brake control valve, and provides unintended consequences.